This invention relates to ion-binding ligands covalently bonded to membranes and to a process for removing and concentrating certain selected ions from solutions using the ligand-membrane compositions, wherein such ions may be admixed with other ions present in much higher concentrations. More particularly, the invention relates to ligand-membrane compositions and to a process for removing such ions from an admixture with other ions in a source solution by forming a complex of the selected ions with the ligand-membrane compositions by flowing such solutions through a contacting device containing the ligand-membrane compositions and then breaking the complex of the selected ion from the composition to which such ion has become attached by flowing a receiving liquid in much smaller volume than the volume of solution passed through the contacting device to remove and concentrate the selected ions in solution in the receiving liquid. The concentrated ions thus removed may then be recovered by known methods.
Composite membranes of the type utilized in one embodiment of the present invention have been previously described in U.S. Pat. No. 4,618,533 to Steuck. Some of the ion-binding ligands of the types disclosed herein are also known. For example, U.S. Pat. No. 4,952,321 to Bradshaw et al. discloses amine-containing hydrocarbons attached to a solid inorganic support such as silica or silica gel wherein the ligand is bound to the solid inorganic support through a hydrocarbon spacer containing a trialkoxysilane group. U.S. Pat. Nos. 5,071,819 and 5,084,430 to Tarbet et al. disclose sulfur and nitrogen-containing hydrocarbons as ion-binding ligands. U.S. Pat. Nos. 4,959,153 and 5,039,419 to Bradshaw et al. disclose sulfur-containing hydrocarbon ligands. U.S. Pat. Nos. 4,943,275 and 5,179,213 to Bradshaw et al. disclose ion-binding crowns and cryptands as ligands. U.S. Pat. No. 5,182,251 to Bruening et al. discloses aminoalkylphosphonic acid-containing hydrocarbons ligands. U.S. Pat. No. 4,960,882 to Bradshaw discloses proton-ionizable macrocyclic ligands. U.S. Pat. No. 5,078,978 to Tarbet et al. discloses pyridine-containing hydrocarbon ligands U.S. Pat. No. 5,244,856 to Bruening et al. discloses polytetraalkylammonium and polytrialkylamine-containing hydrocarbon ligands. U.S. Pat. No. 5,173,470 to Bruening et al. discloses thiol and/or thioether-aralkyl nitrogen-containing hydrocarbon ligands. U.S. Pat. No. 5,190,661 to Bruening et al. discloses sulfur-containing hydrocarbon ligands also containing electron withdrawing groups. Copending application Ser. No. 08/058,437 filed May 7, 1993, discloses oxygen donor macrocycles, for example, ligands containing macrocyclic polyether cryptands, calixarenes, and spherands, multiarmed ethers and mixtures of these. All of these previous reports have involved binding of the ligands to solid inorganic supports via a silane-containing spacer grouping. However, researchers have not previously reported incorporating complex, strongly interacting and highly selective ion-binding ligands into membranes which would be highly desirable because of the high surface-to-area ratios, convenient physical formats, ease of production, ease of use, and inexpensive cost of such membranes. The present invention successfully accomplishes this feat.
The compositions of the present invention comprise ion-binding ligands that are covalently bonded to a membrane through an amide, ester, thioester, carbonyl or other suitable bond. Membranes that are inherently hydrophilic, or partially hydrophilic, and contain moieties appropriate for making these bonds are preferred. Such membranes include polyamides, such as nylon, and cellulosic materials, such as cellulose, regenerated cellulose, cellulose acetate, and nitrocellulose. If the membrane used does not contain reactive groups it may be modified or derivatized appropriately. Composite membranes are also useful. A composite membrane comprises a porous polymer membrane substrate and an insoluble, cross-linked coating deposited thereon. Representative suitable polymers forming the membrane substrate include fluorinated polymers including poly(tetrafluoroethylene) (xe2x80x9cTEFLONxe2x80x9d), polyvinylidene fluoride (PVDF), and the like; polyolefins such as polyethylene, ultra-high molecular weight polyethylene (UPE), polypropylene, polymethylpentene, and the like; polystyrene or substituted polystyrenes; polysulfones such as polysulfone, polyethersulfone, and the like; polyesters including polyethylene terephthalate, polybutylene terephthalate, and the like; polyacrylates and polycarbonates; and vinyl polymers such as polyvinyl chloride and polyacrylonitriles. Copolymers can also be used for forming the polymer membrane substrate, such as copolymers of butadiene and styrene, fluorinated ethylene-propylene copolymer, ethylene-chlorotrifluoroethylene copolymer, and the like.
With composite membranes, the substrate membrane material is not thought to affect the performance of the derivatized membrane and is limited in composition only by its ability to be coated, or have deposited on its surface, an insoluble polymer layer that contains the appropriate reactive group. This provides a hydrophilic layer which interacts well with water or other aqueous solutions. The end result is that when an organic ligand is attached to the surface of either a hydrophilic membrane or a composite membrane having a hydrophilic surface, the basic characteristics of any given ligand molecule are not changed by the process of attaching it to the surface or by the nature of the surface itself.
The coating of composite membranes comprises a polymerized cross-linked monomer. Representative suitable polymerizable monomers include hydroxyalkyl acrylates or methacrylates including 1-hydroxyprop-2-yl acrylate and 2-hydroxyprop-1-yl acrylate, hydroxypropylmethacrylate, 2,3-dihydroxypropyl acrylate, hydroxyethylacrylate, hydroxyethyl methacrylate, and the like, and mixtures thereof. Other polymerizable monomers that can be utilized include acrylic acid, 2-N,N-dimethylaminoethyl methacrylate, sulfoethylmethacrylate and the like, acrylamides, methacrylamides, ethacrylamides, and the like. Other types of hydrophilic coatings that can be used within the scope of the invention include epoxy functional groups such as glycidyl acrylate and methacrylate, primary amines such as aminoethyl methacrylates, and benzyl derivatives such as vinyl benzyl chloride, vinyl benzyl amine, and p-hydroxyvinyl benzene.
The coating of composite membranes also comprises a precipitated crystal system, such as that involving the material known under the trademark xe2x80x9cNAFION.xe2x80x9d xe2x80x9cNAFIONxe2x80x9d is a sulfonic acid or sodium sulfonate of a perfluorinated polyether.
The basic consideration in selecting a composite membrane is that the coating placed on the membrane substrate is the determining factor in defining the chemistry used to covalently attach the ligand. For example, a composite membrane displaying a carboxylic acid functional group can form an amide bond with a pendant amine group from the ligand, one of the most stable methods of ligand immobilization. The composite polymers referenced above can be prepared with carboxylic acid active groups that can be readily converted to amides upon reaction with an amine group on a ligand. However, any of the other organic species which are reactive toward an acid chloride could be used to attach an organic ligand to the surface. Additional examples of such groups would be esters, thioesters, Grignard reagents, and the like.
If the reactive group on the surface is a sulfonic acid, then an analogous procedure using a sulfonyl chloride would yield results similar to those obtained with carboxylic acid functionalities. One such polymer containing sulfonic acid reactive groups is available under the tradename xe2x80x9cNAFIONxe2x80x9d from DuPont as described above.
The ligand is selected from the group consisting of amine-containing hydrocarbons, sulfur and nitrogen-containing hydrocarbons, sulfur-containing hydrocarbons, crowns and cryptands, aminoalkylphosphonic acid-containing hydrocarbons, polyalkylene-polyamine-polycarboxylic acid-containing hydrocarbons, proton-ionizable macrocycles, pyridine-containing hydrocarbons, polytetraalkylammonium and polytrialklylamine-containing hydrocarbons, thiol and/or thioether-aralkyl nitrogen-containing hydrocarbons, sulfur-containing hydrocarbons also containing electron withdrawing groups, and macrocyclic polyether cryptands, wherein the ligands are capable of selectively complexing ions such as either certain alkali, alkaline earth, noble metal, other transition metal, and post transition metal ions when contacted with solutions thereof when admixed with other ions.
The process for removing and concentrating certain selected ions using the ligand-membrane compositions is carried out in any manner that provides for bringing the ion to be removed into contact with the ligand affixed to the membrane. Overall the process comprises selectively removing and concentrating one or more selected species of ion from a plurality of other ions in a multiple ion solution in which the other ions may be present at much higher concentrations. The multiple ion solution or source solution is brought into contact with a composition of the present invention. The preferred embodiment disclosed herein involves carrying out the process by bringing a large volume of the multiple ion solution into contact with a composition of matter of the invention. Contact is preferably made in a contacting device comprising a cartridge containing the composition of matter of the invention by causing the multiple ion solution to flow through the cartridge and thus come in contact with the composition of the invention. However, various contact apparatus may be used instead of a cartridge. The selected ion or ions complex with the composition. Following the complexing step, a small volume of a receiving liquid or eluant is brought into contact with the loaded composition to break the complex by chemical or thermal means and to dissolve the selected ions and carry them away from the composition. The selected ions can then be recovered from the receiving liquid by well known procedures.
More particularly, the process comprises forming a complexing agent by covalent bonding of a ligand of the type mentioned previously to a composite membrane, such as one of those previously mentioned. The complexing agent is then introduced into a contacting device such as a cartridge. The solution containing the multiple ion species flows through the cartridge in contact with the complexing agent, whereby the selected ions complex with the complexing agent. The selected ions are thus separated from the rest of the ion mixture that flows out of the cartridge. A small volume of the receiving liquid or eluant is then passed through the cartridge to break the complex and dissolve and carry out of the cartridge the selected ion or ions. The selected ions are then recovered from the receiving phase by well known procedures.
Preparation of the Ligand-Membrane Compositions
The compositions of the present invention may be prepared by any suitable method wherein the ligands can be covalently bonded to a membrane containing reactive functional groups.
The membrane is selected to yield both selected bulk properties and selected surface properties. For naturally hydrophilic membranes, the selected bulk and surface properties will be provided by whatever polymer that comprises the membrane. For composite membranes, the selected bulk properties will be provided by the membrane substrate and the selected surface properties will be provided by the coating. A composite membrane is formed by depositing a monomer directly on the surface of the substrate, including the inner surfaces of the pores, by in situ deposition of the cross-linked monomer. The desired deposition of the cross-linked monomer onto the porous substrate is effected as a direct coating and does not require or utilize an intermediate binding chemical moiety. Any monomer for the coating polymer can be used so long as it is capable of being polymerized by free radical polymerization and can be cross-linked. The only requirements of the polymerized monomer is that it is capable of coating the entire surface of the porous membrane, that it provide the surface with ligand-reactive functional groups, and that it be sufficiently hydrophilic to allow for efficient use of the ligand to be attached. Generally, the porous substrate has an average pore size between about 0.001 and 10 xcexcm, and more usually, between about 0.1 and 5.0 xcexcm. The composite membrane is formed by any suitable method, such as is disclosed in U.S. Pat. No. 4,618,533, which is hereby incorporated by reference. Briefly, this procedure involves washing the porous membrane substrate with a suitable solvent for wetting the entire surface of the substrate. The substrate is then bathed in a mixture of the free radical polymerizable monomer, a polymerization initiator, and a cross-linking agent in a solvent under conditions to effect free radical polymerization of the monomer and coating of the porous substrate with the cross-linked polymer. The surface of the coated polymer membrane contains hydrophilic or polar-substituents that can be activated to react with and covalently bond the ligands to the membrane surface.
The composite membranes prepared according to U.S. Pat. No. 4,618,533 can contain carboxylic acid moieties on the surface. Other suitable moieties could include hydroxyl, sulfonic acid, epoxy, primary amine, and derivatized benzyl groups such as polymers referenced above.
Preparation of a composite membrane by a precipitated crystal technique involves, briefly, washing the porous membrane substrate with a suitable solvent for wetting the entire surface of the substrate. The substrate is then bathed in a solution containing the crystals that are to be precipitated. This solution is then removed and the membrane substrate is treated with a compound that precipitates and fixes the crystals to the substrate. The membrane is washed and dried before use.
In the present invention, the activation of the carboxylic acid groups is exemplified by the reaction of the carboxylic acid groups with thionyl chloride to form acid chloride groups according to the formula:
membrane-COOH+S(O)Cl2xe2x86x92membrane-C(O)Cl+SO2+HCl 
Carboxylic acid groups also can be converted to acid chloride groups by reaction with phosphorus pentachloride or phosphorus trichloride.
Ligands (L) containing reactive amines, alcohols, thiols, Grignard reagents and the like may be covalently bonded to the membrane through the xe2x80x94C(O)Cl group as follows:
membrane-C(O)Cl+H2NLxe2x86x92membrane-C(O)NHL+HCl (amide)xe2x80x83xe2x80x83(1) 
membrane-C(O)Cl+HOLxe2x86x92membrane-C(O)OL+HCl (ester)xe2x80x83xe2x80x83(2) 
membrane-C(O)Cl+HSLxe2x86x92membrane-C(O)SL+HCl (thioester)xe2x80x83xe2x80x83(3) 
membrane-C(O)Cl+XMgLxe2x86x92membrane-C(O)L+MgXCl (ketone)xe2x80x83xe2x80x83(4) 
In a similar manner, the activation of the sulfonic acid groups is exemplified by the reaction of the sulfonic acid groups with thionyl chloride to form sulfonyl chloride groups according to the formula:
membrane-S(O)2OH+S(O)Cl2xe2x86x92membrane-S(O)2Cl+SO2+HCl 
Sulfonyl chloride groups also can be obtained by reaction of sulfonic acid groups with phosphorus pentachloride or phosphorus trichloride.
Ligands containing reactive amines, alcohols and the like may be covalently bonded to the membrane through the xe2x80x94S(O)2Cl group as follows:
membrane-S(O)2Cl+H2nLxe2x86x92membrane-S(O)2NHL+HCl (sulfonamide)xe2x80x83xe2x80x83(1) 
membrane-S(O)2Cl+HOLxe2x86x92membrane-S(O)2OL+HCl (sulfonate ester)xe2x80x83xe2x80x83(2) 
This reaction does not proceed as readily as the reactions with acid chlorides formed from carboxylic acids. However, any reaction may be used provided it is functional to form a stable covalent bond between the ligand and the membrane. For the present, it has been found that the amide linkage is most stable and readily formed.
Ligands, which may be adapted to contain xe2x80x94NH, xe2x80x94OH, xe2x80x94SH, xe2x80x94MgX moieties which are reactive so as to form a covalent bond with membrane attached functionalities are illustrated in the patents indicated below, which are hereby incorporated by reference: amine-containing hydrocarbons (U.S. Pat. No. 4,952,321), sulfur and nitrogen-containing hydrocarbon ligands (U.S. Pat. Nos. 5,071,819 and 5,084,430), sulfur-containing hydrocarbon ligands (U.S. Pat. Nos. 4,959,153 and 5,039,419), crowns and cryptand ligands (U.S. Pat. Nos. 4,943,375 and 5,179,213), aminoalkylphosphonic acid-containing hydrocarbon ligands (U.S. Pat. No. 5,182,251), proton-ionizable macrocycle ligands (U.S. Pat. No. 4,960,882), pyridine-containing hydrocarbon ligands (U.S. Pat. No. 5,078,978), polytetraalkylammonium and polytrialkylamine-containing hydrocarbon ligands (U.S. Pat. No. 5,244,856), thiol and/or thioether-aralkyl nitrogen-containing hydrocarbon ligands (U.S. Pat. No. 5,173,470), and sulfur and electron withdrawing group-containing hydrocarbon ligands (U.S. Pat. No. 5,190,661).
An oxygen donor macrocycle ligand, such as disclosed in copending application Ser. No. 08/058,437 filed May 7, 1993, having a reactive grouping attached, may be prepared by various reaction schemes. Two are illustrated. The first involves the reaction of a cis dihydroxy crown ether with a polyether diol wherein the diol groups have been activated by reaction with a xe2x80x9cleavingxe2x80x9d group such as tosyl chloride. The following reaction sequence (Reaction A) shows the formation of an oxygen donor macrocycle ligand (Formula 2) by means of reacting a cis dihydroxy crown ether (Formula 3) with a tosylated polyether diol (Formula 4) as follows wherein Ts stand for the tosyl group, R3, R4, R5, and R6 is each a member independently selected from the group consisting of H, allyloxymethyl, alkylthio, alkylamino, carboxy, carboxyalkyl, and epoxyalkyl. R7 is a member selected from the group consisting of H and alkyl, Z is a member selected from the group consisting of o-phenylene and o-naphthalene or alkyl, R1 and R2 are each a member selected from the group consisting of H, allyl, alkenyl, carboxy, carboxyalkyl, allyloxy, aminoalkyl, hydroxy, thio, and alkylthio. The functional groups that are not directly reactive with the corresponding groups on the surface of the membrane must be further reacted so as to allow a covalent bond. As an example, a carboxy alkyl functional group could be converted to an acid chloride and further reacted with ethylene diamine (in large excess) to provide a mono amide with a free amine. This could then be reacted with the membrane. Further, n is an integer of 2 to 4, a is an integer of 0 or 1, b is an integer of 0 to 3 with the proviso that b must be at least 1 when a is 0, and m is an integer of 0 to 5. To provide a reactive grouping to react with a reactive membrane, it is mandatory that one or two, and preferably only one, of the R1 through R5 groups must be other than H. The remaining R1 through R5 groups are H. 
While the Ts or tosyl group is illustrated above, other leaving groups such as mesylates, chlorides, bromides and the like can also be utilized. The tosyl group is preferred because it is crystalline and has better reaction properties.
The second reaction scheme involves the reaction of a cis dibromomethyl crown ether with a polyether diol. The following reaction sequence (Reaction B) shows the formation of an oxygen donor macrocycle ligand (Formula 2) by means of reacting a cis dibromomethyl crown ether (Formula 5) with a polyether diol (Formula 6) as follows wherein symbols have the same meaning as given for Formula 2 above: 
The compound corresponding to Formula 2, having a reactive grouping may then be reacted with a membrane derivatized with hydrophilic functionalities.
Polyalkylene-polyamine-polycarboxylic acid-containing hydrocarbon ligands may be prepared by various methods. For example, in one method the polyalkylene-polyamine-polycarboxylic acid ligand is bound to the membrane. In a second method, a polyalkylenepolyamine is reacted with a membrane followed by reacting with a polycarboxylic acid.
The above described ligands have heretofore been attached to solid supports such as silica gel, silica, glass, glass fibers, nickel oxide, zirconia, alumina, titania and the like. The attachment of the ligand to the solid support has been by means of a silane spacer grouping. There are certain drawbacks to the use of such solid support. For example, they most often have to be contained in a column or similar structure and do not have the adaptability for other configurations that a membrane possesses. Further, silane chemistry is complicated and limits certain reactions or applications. Finally, the instability or even partial dissolution of the inorganic supports in some solution matrices makes their use in some separation applications poor or unacceptable. However, such ligands, that have been attached to the above mentioned inorganic solid supports, have not previously been affixed to membranes.
The novelty of the invention is in the membrane ligand combination and in the method of using such combinations in removing desired ions. Any of the ligands previously used may be modified for use in the present invention. Because the ligands are not in and of themselves novel, they will be referred to as ligands (xe2x80x9cLxe2x80x9d) and may be further designated by classes, i.e. amine-containing hydrocarbon ligands; sulfur and nitrogen-containing hydrocarbon ligands; sulfur-containing hydrocarbon ligands; crown and cryptand ligands; aminoalkylphosphonic acid-containing hydrocarbon ligands; proton-ionizable macrocycle ligands; pyridine-containing hydrocarbon ligands; polytetraalkylammonium and polytrialkylamine-containing hydrocarbon ligands; thiol and/or thioether-aralkyl nitrogen-containing hydrocarbon ligands; sulfur and electron withdrawing group-containing hydrocarbon ligand; and oxygen donor macrocycle ligands. This listing of ligands is exemplary only and is not intended to be all encompassing. Other ligands, known or yet to be developed, may also be utilized with the only limitation being that they can be covalently attached to the membrane and are functional in the selective attracting and binding of the selected ions being removed from the solutions being treated.
The membrane ligand combination of the invention can therefore be defined by the formula:
M-B-L 
wherein M is any membrane or composite membrane derivatized to have a hydrophilic surface and contain polar functional groups, L is any ligand as defined above containing a functional grouping reactive with an activated polar group from the membrane and B is the covalent linkage formed by the reaction between the activated polar group and the functional group of the ligand. Representative of 3 linkages are members selected from the group consisting of amide (NHCO), ester (COO), thioester (COS), carbonyl (CO), ether (O), thioether (S), and sulfonamide (SO2NH).
The membrane/ligand compositions of the present invention that are useful for separating selected ions will be apparent to those skilled in the art by the following examples each of which utilizes a composite membrane prepared according to U.S. Pat. No. 4,618,533 and containing carboxylic acid groups or sulfonic acid groups.